This invention relates to a selective call radio network for selectively calling a plurality of mobile units which may be pagers or beepers and are called by call radio signals of different bit rates. This invention relates also to a central station and to each of base stations used in the selective call radio network.
This invention relates to an earlier patent application filed by Hironao Tanaka, the present inventor, on May 17, 1996, as patent application Ser. No. 08/648,974.
In the selective call radio network, the base stations are connected to the central station by wired and/or radio channels and altogether have an overall area which comprises a plurality of service areas and in which the mobile units are independently movable. The base stations have the service areas, respectively, two adjacent ones of which have an overlapping area. The call radio signals are transmitted from the base stations to their respective service areas in a predetermined format, such as the POCSAG (Post Office Code Standardisation Advisory Group, the United Kingdom) format, or radio carriers of a common signal frame which is typically 30 seconds long for use in the present invention.
The central station is connected to a communication network which may be a public telephone network and includes a great number of subscriber sets. In the manner which will later become clear, the central station comprises in general a request receiving unit for receiving from the communication network a call request for a selected one of the mobile units as a transmission request of the predetermined format and an announcing device for announcing the transmission request to the base stations in an announced signal. Each base station comprises a signal receiving unit for receiving the announced signal as a received signal specifying the transmission request as a received request and a transmitting device for transmitting to its individual service area the received request in one of the call radio signals. If present in the individual service area, the selected one of mobile units responds to this one of the call radio signals.
In such a selective call radio network, the base stations transmit the call radio signals in a common radio channel to enable the selected one of mobile units to respond in whichever of the service areas. Use of the common radio channel, however, may give rise to an interference between two of the call radio signals in the overlapping area. For example, selective call of the mobile units fails with an appreciably high probability when these two of the call radio signals reach the overlapping area with a phase difference. Consequently, various schemes have already been proposed to make the base stations transmit the call radio signals in phase coincidence.
One of such schemes is disclosed in Japanese Patent Prepublicaion (A) No. 311,088 of 1994. In the selective call radio network of this patent prepublication, a phase reference station is used in transmitting a radio phase reference signal to the base station and in making the base stations thereby adjust transmission timings of the call radio signals to establish the phase coincidence even in the overlapping area.
Another of the schemes is revealed in Japanese Patent Prepublication (A) No. 291,713 of 1994 for a patent application filed by the present assignee in Japan. According to this scheme, use is made of a satellite radio signal emitted by a GPS (global positioning system) satellite. Each of the central station and the base station comprises a GPS receiver for extracting a GPS timing signal from the satellite radio signal. The central and the base stations comprise timing signal generators responsive to the GPS timing signal for generating central and base timing signals of an identical timing. From the central station, the announced signal is announced to the base stations with a transmission start time instant indicated therein for the call radio signal based on the central timing signal. The transmission start time instant is decided to be later than an announcing time instant of announcement of the announced signal from the central station by a time delay which is not shorter than an arithmetic sum of a longest propagation time interval between the central station and a remotest base station and a maximum processing time interval of processing the received signal into the call radio signal in the base stations. Each base station processes the received signal by forming a queue of received requests in successive received signals to begin transmission of the call radio signal at the transmission time instant based on the base timing signal. In this manner, the phase coincidence of the call radio signals is insured at the base stations.
Instead of using the satellite radio signal of the GPS satellite, use is possible of a method and network disclosed in U.S. Pat. No. 5,402,424 issued to Yukari Kou. Kou patent will be described far later in the description and is incorporated herein by reference. Another network is revealed in U.S. Pat. No. 4,800,560 issued to Takahiro Aoki and another and is incorporated herein by reference.
It is necessary in such a selective call radio network to use only a limited number of radio channels for the great number of mobile units. It has therefore been planned to give a higher bit rate to the call radio signal, such as 1,200 bps (bits per second) instead of 512 bps. This has resulted in use of mobile units of various bit rates in a single selective call radio network. In other words, each base station must transmit the call radio signals of various bit rates. As a result, the phase coincidence must be very strict for high bit rate call radio signals. Despite this necessity, only one bit rate is used in the schemes proposed in the patent prepublications cited above.
Assignment of one radio channel to the call radio signals of each bit rate is possible on transmitting such call radio signals with different bit rates. It should, however, be noted in this connection that traffic of calls not necessarily be uniform over the different bit rates and that the traffic may vary or be biassed with time. This would result in an unfavorable state such that the radio channel of a low bit rate may not be so busy at a time while the radio channel of a high bit rate may fall into overflow in the meanwhile and that the radio channels are not used with an excellent efficiency. Alternatively, it is possible to assign the different bit rates to signal frames according to the call radio signals included in the signal frames. This would result, when the signal frames have a common frame length predetermined as in the POCSAG format by a period of queues of the call radio signals, in another unfavorable state such that the signal frames unavoidably have an idle duration as a result of fluctuations dependent on the bit rates in the traffic and that effective use of the radio channels is difficult.
It may be possible to use a variable frame length depending on the bit rates by using different formats other than the POCSAG format. This, however, results in difficulty in specifying the transmission start time instants of the respective signal frames as precisely discrete time intervals shorter than one second when the transmission time instants are specified by the central station. Moreover, various restrictions give rise to another difficulty in fulling up the idle duration between two successive signal frames to a sufficient extent. If a short frame length were used in consideration of the traffic dependent on the bit rates, an increase would be inevitable in rates of the idle duration to such short signal frames. This again reduces the efficiency of utilizing the radio channels.
A technology proposed in Japanese Patent Prepublication (A) No. 227,135 of 1988 is useful in coping with the fluctuations in the traffic with the common frame length used for various bit rates. According to this technique, each signal frame comprises a plurality of subframes equal in number to groups into which the mobile units are classified in accordance with the different bit rates. The signal frame further comprises a frame and bit synchronization pattern for the subframes. It is, however, described in this patent prepublication that the mobile units must comprise call radio signal receivers specific to the groups of different bit rates.